Nickel alloy

ABSTRACT

The invention relates to a nickel alloy derived from René 125, but with reduced levels of certain elements (Zr, B, P, S, Si and, to a lesser extent, Ti and Hf) in order to limit the appearance of cracks upon solidification in a moulding process. Specifically, 4.80%≦Al≦5.00%, 1.48%≦Hf≦1.52%, 2.28%≦Ti≦2.33%, 0.005%≦B≦0.01%, 1.77%≦Mo≦1.97%, and Zr≦0.007%. Other elements can have levels that match those of René 125.

The subject of the invention is a nickel alloy.

It has been conceived to improve the foundry manufacture of certain parts of complex shape such as distributor blades used in engine stators in aeronautics.

These parts are constituted of blades fitted into platforms, the assembly of which on an engine constitutes a ring. Certain of these parts are traditionally constructed from a nickel alloy known as René 125, but which is particularly liable to produce cracks on moulding, during the solidification of the molten metal, which can constitute an important cause of rejects.

An alloy of composition analogous to that of René 125 was thus searched for in order to conserve for the main part the favourable properties of this alloy, but which would be much less prone to the formation of cracks.

According to the invention, the alloy proposed here comprises, in a nickel base, from 9.5% to 9.90% by weight of cobalt, 8.70% to 9.00% by weight of chromium, 6.65% to 7.05% by weight of tungsten, 3.67% to 3.87% by weight of tantalum, 1.77% to 1.97% by weight of molybdenum, 0.10% to 0.12% by weight of carbon; and it is differs from the conventional composition of René 125 by levels of 4.80% to 5.00% by weight of aluminium, 1.48% to 1.52% by weight of hafnium, 2.28% to 2.33% by weight of titanium, 0.0005% to 0.01% by weight of boron. The alloy does not in principle comprise other ingredients, except at much lower levels than those mentioned until now, or only present as impurities. It is thus underlined in particular that zirconium, quite abundant in René 125, is eliminated entirely or almost entirely from the present alloy (at the most 0.007% by weight), as are phosphorous and sulphur (at the most 0.001% each by weight). Certain other criteria, corresponding to secondary aspects of the invention, can still advantageously be respected as will be detailed hereafter.

This alloy resists the hot formation of cracks much better than René 125; it may thus be employed in processes of foundry manufacturing parts of complex shapes.

The table below indicates the respective compositions of the alloy René 125 (in nominal values) and of the alloy according to the invention, in minimum values and maximum values. Only significant elements have been reported, others being in general present as impurities.

TABLE (% by weight) Element Co Cr W Al Ta Ti Mo Hf C Zr B P S René 125 10.00 9.00 7.00 4.80 3.80 2.50 2.00 1.55 0.09 0.05 0.015 <0.01 <0.075 Invention 9.50 8.70 6.65 4.80 3.67 2.28 1.77 1.48 0.10 0 0.005 0 0 (min) Invention 9.90 9.00 7.05 5.00 3.87 2.33 1.97 1.52 0.12 0.007 0.010 0.001 0.001 (max)

Several indications on the effects obtained are given below. It appeared during tests that boron, zirconium, silicon, sulphur, phosphorous, hafnium and titanium were favourable to the appearance of cracks while hot, and this is why the levels of these elements are reduced compared to the composition of the starting alloy René 125. From a quantitative viewpoint, titanium and hafnium saw their levels the most reduced, and zirconium, phosphorous and sulphur have become elements present only as traces, whereas zirconium was necessarily present and at a non-negligible level in the starting alloy. The level of certain other elements, which are not incriminated as favouring the appearance of cracks, has also been reduced significantly in order to increase the level in the nickel base, since it has been noted that cracks were less numerous with a high nickel level.

The favourable effect of the reduction or the quasi-elimination of these elements may be explained in this way: the particles that they form accumulate at the joints of grains of the alloy during solidification. The internal stresses that develop upon solidification tend to produce fissuring at the grain joints, and all the more readily while the latter are still molten. The reduction in the level of these elements favours the solidification of the grain joints at temperatures closer to that of the grains and thus enhances the cohesion of the alloy.

More detailed indications on the different elements are given hereafter.

Boron and zirconium: it appeared that these elements were those that most thwarted solidification at the grain joints, and were thus responsible for solidification at a temperature well below that of the grains themselves in René 125. Their level is thus considerably reduced or even eliminated in the alloy of the invention.

As regards boron, much lower probabilities of cracks (four times lower or even less) than in René 125 have nevertheless been observed at the levels proposed in the invention, so that the total elimination of this element is not recommended.

Phosphorous and sulphur merit the same commentaries, but their importance is less since their level is already reduced in René 125.

Titanium and hafnium: their effect was the same but less important, such that their total elimination is not recommended, a slight reduction in the level sufficing to reduce enormously the probabilities of cracks. In the same way as for boron, the levels proposed in the invention led to probabilities of cracks at least four times less than in René 125 (the comparative tests each time related to a single variant element).

Total level of hafnium, titanium and aluminium: the reject rate had a low levelling off between around 8.73% and 8.77% (total of levels), then increased sharply, then being multiplied by at least around four. Aluminium does not have in itself the harmful role of the other elements mentioned above and its level is even increased in the invention, but this criterion shows that it cannot be in excess. The total level is advantageously below the latter figure.

Nickel: the reject percentage of parts due to cracks dropped from a level of 59.71% of nickel, and reached a levelling off corresponding to very low reject rates from around 59.83%. The level is advantageously above the latter figure.

Silicon: it is at the most 0.10% by weight in René 125, and advantageously present as traces in the invention.

The heat treatment used for the tests as for the manufacture by moulding of the bladed distributors was a T3R, namely a heating to 1175° Celsius for 30 minutes, then a cooling to 1095° Celsius in 6 to 10 minutes, then a cooling in the oven to 650° Celsius, then a cooling in air, and an annealing at 815° Celsius for 16 hours under vacuum or protective atmosphere. 

1. A nickel alloy, comprising: nickel, 9.50% to 9.90% by weight of cobalt, 8.70% to 9.00% by weight of chromium, 6.65% to 7.05% by weight of tungsten, 3.67% to 3.87% by weight of tantalum, 0.10% to 0.12% by weight of carbon, 1.77% to 1.97% by weight of molybdenum, 4.80% to 5.00% by weight of aluminium, 1.48% to 1.52% by weight of hafnium, 2.28% to 2.33% by weight of titanium, 0.005% to 0.01% by weight of boron, and less than 0.007% by weight of zirconium.
 2. The nickel alloy according to claim 1, further comprising: less than 0.001% by weight of phosphorous, and less than 0.001% by weight of sulphur.
 3. The nickel alloy according to claim 1, comprising: more than 59.83% by weight of nickel.
 4. The nickel alloy according to claim 1, wherein a total weight of titanium, hafnium and aluminium is less than 8.77%.
 5. The nickel alloy according to claim 1, wherein the nickel alloy is suitable for manufacturing a bladed distributor part of an aeronautics engine stator.
 6. The nickel alloy according to claim 5, wherein the nickel alloy is treated with a T3R heat treatment. 